Geoscience Reference
In-Depth Information
to rise up in great gas bubbles to the surface. A sudden fall in sea level can also destabil-
ize gas hydrates so that they release their methane, which is a powerful greenhouse gas.
It is probable that a sudden global warming 55 million years ago was caused by methane
released from gas hydrates. It has even been suggested that some accounts of ships lost in
the imaginary Bermuda Triangle in recent times originated from descriptions of large gas
bubbles breaking the surface, capsizing boats or asphyxiating their crew.
Large quantities of organic material can become buried in ocean sediments and can, in the
right circumstances, get turned into oil. That tends to happen in shallow marine basins that
are undergoing crustal stretching. This thins the crust, deepening the basin so that it fills up
with more sediment. But at the same time the organic material gets buried deeper, closer to
the internal heat of the mantle so that it is cooked into crude oil and natural gas. This can
then rise up through permeable strata and collect beneath impervious clays or salt layers.
Rock salt is particularly mobile as it is not very dense and tends to rise up through strata in
big domes. Often these can trap rich oil and gas deposits, as happens in the Gulf of Mexico.
Life underground
But not all the organic material in ocean sediments is dead. Living bacteria are often abund-
ant in sediments more than 1,000 metres under the sea floor, in rocks over a hundred mil-
lion years old. It seems likely that they were living in the sea floor mud and remained as
it was buried deeper and deeper all that time ago. They don't exactly lead exciting lives
but they're certainly not dead. It is estimated that they may divide only once every 1,000
years and live by anaerobically digesting organic material and releasing methane. Some
bacteria can also survive at high temperatures, possibly up to the 100 to 150 degrees Celsi-
us at which oil forms, and they may play a significant part in this process. It is possible that
90% of all terrestrial bacteria live underground and together comprise as much as 20% of
the total biomass on Earth.
The longest mountain chain on Earth
If you were to drain the water from the world's oceans and reveal the spectacular landscape
down there, the biggest feature would not be the great ocean island mountains taller than
Everest or the great chasms that dwarf the Grand Canyon, it would be a mountain chain
70,000 kilometres in length: the mid-ocean ridge system. The ridges run around the planet
like the seam in a tennis ball. Peppered along their length are volcanic fissures. Sometimes
these erupt slowly underwater, producing pillow-shaped clumps of dense black basalt lava,
like toothpaste from a tube. These are the zones of creation where new ocean crust is form-
ing as the sea floor spreads.
